As vehicle or mobile cranes of the kind set forth in the opening part of this specification can also travel on public roads, they must comply with the respective road traffic regulations. That involves in particular complying with the permissible axle loads for on-road travel. Thus for example the permissible axle load in Germany is 12 tonnes. Admittedly some countries, for example the Netherlands, allow higher permissible axle loads—thus for example an axle load of 13 tonnes is allowed in the Netherlands—the permissible laden weight of a vehicle crane is nonetheless however generally limited by the respectively permissible axle loads.
In order not to exceed the permissible axle loads in spite of an increase in the total laden weight of a vehicle crane, for example by virtue of transporting ancillary equipment and by virtue of the counterweights required for a given use, it is already known for a so-called additional axle to be releasably mounted to the rear of the chassis or running gear of the vehicle crane. The fitment of such an additional axle can ensure that the total laden weight of the vehicle crane which is increased as a consequence of ancillary equipment and an additional counterweight is distributed to a further axle, whereby the loading per axle overall can be further reduced. By virtue of mounting such an additional axle it is then possible for example to travel to a location of use just with the vehicle crane alone and without further transport vehicles which are normally required for transporting ancillary equipment and possibly an additional counterweight, as the chassis which is modified in that way can carry more weight, as a consequence of the additional axle.
An additional axle of the kind which is already known should—but does not necessarily have to—be integrated completely into the braking, suspension and steering system of the chassis of the vehicle crane. Then no downgrading in terms of steering angle, braking or suspension system has to be accepted by virtue of integration of the additional axle into the maneuvering system of the vehicle crane itself. If however the additional axle is not coupled to the braking, suspension and steering system of the chassis of the vehicle crane, the effect of that can be that the braking and/or suspension travels of the vehicle crane are worsened and the turning circle also increases, as a consequence of the total laden weight of the vehicle crane which is increased by any ancillary equipment and additional counterweight.
In DE 101 12 084 A1 a mobile thick matter pump is disclosed comprising a hydraulically operated support construction, a distribution boom and a pneumatic cushioned trailing axle. To ensure stability during the pump operation, the trailing axle can be lifted by means of a lifting element by bleeding the pneumatic suspension, whereby the bleeding of the pneumatic suspension and the operation of the lifting element and a locking element can be controlled according to the on-condition of the auxiliary drive of the vehicle motor, used to supply the compressed oil of the drive hydraulics. It is mentioned that a trailing axle may be constructed to be steerable.
In DE 28 37 398 A1 a multi-axle vehicle crane is disclosed having at least one active steered axle. The multi-axle vehicle comprises a trailing or module axle, respectively which can be fitted to the chassis of the vehicle crane. This known module axle and trailing module, respectively comprises at least one steerable axle. The steering gear of this steering axle is coupled to the steering gear of the chassis of the vehicle crane.
A trailerable load-transferring assembly as shown in US 2004/0007840 A1 is releasably attachable to the rear portion of a load-bearing vehicle. The load-transferring assembly includes a pivot connection, an auxiliary axle, an elongate abutment member that extends between a free end and an abutment end, and an actuator assembly disposed between the free end of the abutment member and the auxiliary axle. When the load-transferring assembly is connected to the vehicle by the pivot connection, the actuator assembly is actuated such that it applies a lifting force on the free end relative to the auxiliary axle. The auxiliary axle is configured as a self steering axle. It is mentioned, that in some embodiments, it may be desirable to lock the wheels of the auxiliary axle in a position suitable for straight movement in reverse.
An additional axle as mentioned above which is not integrated into the braking, suspension and steering system of the chassis is known for example from DE 197 28 822 A1 which is referred to therein as an attachment trailing chassis arrangement. The attachment trailing chassis arrangement described therein is specifically calibrated in respect of its weight distribution, so that, utilising the coupling and locking means provided thereon, it can be attached to the chassis of a vehicle crane using the lever assembly thereof, and if necessary can be released therefrom again. However, as has already been mentioned above, that attachment trailing chassis arrangement is found to be disadvantageous by virtue of the fact that it is not integrated into the suspension system of the chassis of the vehicle crane. As therefore the trailing chassis arrangement is not integrated either into the braking system or into the suspension system of the vehicle crane, that additional axle cannot be utilised for dynamic axle load compensation. A further disadvantage of the attachment trailing chassis arrangement described in DE 197 27 822 A1 is also that the axle thereof cannot be steered actively by way of the steering circuit of the chassis of the vehicle crane. Rather, that attachment trailing chassis arrangement only has a slight trailing action with limited lock angles, whereby the axle is itself passively steered only to a slight degree by steering movements initiated by way of the steering arrangement of the chassis of the vehicle crane. Active steering which could overall positively influence the turning capability of the vehicle crane is however not provided in the case of that attachment trailing chassis arrangement.
A further disadvantage of steered axles of multi-axle vehicle cranes is that the current safety regulations require that each steered axle, in the event of a failure, still remains controllable for example by way of an emergency steering circuit. Thus the chassis arrangement of a vehicle crane usually includes a main steering circuit which in turn has a first and a second steering circuit. In that case, the one steering cylinder of the respective steered axle is controlled by way of the first steering circuit and the other steering cylinder of the respective steered axle is controlled by way of the second steering circuit. In the event that now either the first and/or the second steering circuit should fail, the axle should still continue to be controllable by way of the emergency steering circuit, insofar as for example an emergency steering pump pumps hydraulic oil into the respectively defective steering circuit. However the installation of an emergency steering circuit in a vehicle crane, the implementation of which has already been effected by Terex-Demag GmbH & Co KG/Germany under the name ‘Fail Operational System’ is very cost-intensive and complicated and in addition can result in an unwanted increase in the total laden weight of the vehicle crane which, as is known, is to be kept low.
If therefore the attempt were made to also steer a separate additional axle which can be mounted releasably to the chassis in an active mode by way of the steering system of the chassis of the vehicle crane, it would then be necessary, for safety reasons, for that additional axle to be steered not only by way of the main steering circuit of the chassis but, to cover the event of a failure of the main steering circuit, to provide at least one emergency steering circuit with which the additional axle would remain steerable even in a fault situation. Admittedly, the provision of an emergency steering circuit for all steered axles of a chassis of a vehicle crane, as explained, is already known, but, so that an additional axle could also be reliably steered actively together with the running gear of the chassis, the additional axle would have to be suitably converted so that in the normal situation it also remains reliably controllable by way of the main steering circuit and in the event of failure thereof it remains controllable by way of the emergency steering circuit which then comes into action. Admittedly such a control configuration for the additional axle by way of an emergency steering circuit has already been developed by Terex-Demag GmbH & Co KG/Germany under the term ‘Fail Operational System’ and has also been implemented, but that system not only affords the desired advantages but also involves the above-described disadvantages such as for example an increase in total laden weight. In addition the interface for electro-hydraulic coupling of the additional axle to the chassis of the vehicle crane, which is required for active steering of the additional axle, is a constant weak point, the disadvantages of which have to be eliminated as far as possible.